Abstract
The intestinal microbiota in finfish and shellfish is involved in a wide range of biological processes that benefit the host. In this respect, genus Bacillus is highly represented, and approximately 25 species have been detected in the digestive tract of numerous finfish and shellfish species. In the early studies of the gut microbiota of finfish and shellfish, cultured methods were used, for example, Bacillus, but due to limitations, culture-independent techniques have been used during the last decade. Studies conducted on the gut microbiome composition using amplicon sequencing have also confirmed the abundance of Firmicutes indicating bacilli as one of the major gut-associated bacteria in diverse fish species. Evaluation of Bacillus species as a part of the gut microbiota of finfish and shellfish species is of importance as they have great potential applications in aquaculture as they can form the spores that help them survive in the harsh environmental conditions. According to the available literature, Bacillus are one of the most studied probiotics in aquaculture, as they are non-pathogenic and non-toxic to aquacultural environments and animals. In this chapter, we present information on the presence of Bacillus in digestive tract of finfish and shellfish species and some general characteristic information on their ability of exo-enzyme production, production of antimicrobial substances to inhibit in vitro growth of pathogens, making them suitable candidates compared to other probiotics.